Here's what I believe is the driver in the TH-50 (MTX 9515)
Here's one with a custom finish

Here's the specs for hornresp

Here's the published response

Here's hornresp's prediction, using my 'guestimate' of the folding as pictured in post one, with the MTX driver. Looks pretty close I'd say - note the dips and peaks correspond.

Here's how you get the price down from $600 or so to half the cost. Alpine SWR-1522D is a car audio driver that seems to be exceptionally affordable if you buy it online. You might consider the Alpine because there doesn't appear to be another driver available in the world that combines this much output, at this price, with a flat BL curve. (IE there are a handful of drivers that put out more, such as the FI infinite baffle drivers, but they don't have a flat BL curve and the qts is too high for such a 'small' tapped horn.)

Here's the simulated response of the Alpine versus the MTX. The Alpine is a little bit smoother.

*** In order to get this response, I had to lower the depth of the enclosure ***
I reduced the enclosure volume by exactly one third. So instead of being 25.5" deep, the enclosure is 17" deep. In all other respects, the tapped horn is identical to the TH-50. Same folding, same length, same throat, same mouth, etc. Identical.

To get the Alpine driver, just look on Google products. (Not sure if we're allowed to post links to resellers. It is currently available for less than $200, including postage.)

In the impedance plots, we see it's basically identical for both drivers.

The group delay of the Alpine is a little bit cleaner at the lowest frequencies

Excursion of the two drivers is basically identical, because they're both 15s. The MTX will get a bit louder, as it has about 50% more excursion. (That should be good for about 2 more dB of output.) But you can fit three of the Alpine tapped horns in the same footprint as two of the TH-50s would take, and the cost would still be lower, because the Alpine is selling for less than $200 per driver, while the MTX is more than twice as much. If you can find it. (It's been discontinued.)

"Among the duties he assumed when he joined HGU in 2002, Dong-wook Seo is the man responsible for the Hyoam Anex Hall and its concert sound system – and recently undertook an upgrade of the main front-of-house loudspeaker system with help from Seoul-based contractor Media House. ‘The old system gave very low intelligibility for speech,’ Mr Seo explains, ‘but the new system is really fit for a house of worship. It is a totally different sound.’

The system is based on Danley Sound Labs line array loudspeakers, Crown amplification, and Dolby Lake processing, with the 48-channel Midas Venice console that had been installed as part of the previous year’s budget retained for mixing FOH and monitors. Specifically the system uses two Danley SH50 and DFA full-range, point-source cabinets, with a single TH-115 subwoofer on each side of the stage."

This would be a nice affordable build, I am currently without a sub and as I work for a living, (poor) I am all about the lowest cost best performance. I currently have wood cut for 2 of Art's Keystones but no money for drivers atm .
The plan you posted is kinda rough, I will draw it up in sketchup to figure out how to build and post here, any more details would help